Lawyers for a former Naval Academy instructor who faces court-martial in January on charges of lying about a sexual affair with two midshipmen are accusing prosecuting attorneys of misconduct, alleging they violated ethics rules by denying a key witness the right to counsel.

At issue is a grant of immunity for another former Naval Academy instructor. Prosecutors say Maj. Michael Pretus and the defendant, Maj. Mark Thompson, participated in a 2011 threesome with a female student while Thompson was an instructor and Pretus was a guest at the academy, there to lecture about his combat experiences.

During a 2013 court-martial in which Thompson was accused of indecent conduct and sexual assault, Pretus vouched for Thompson as a witness for the defense. Pretus was investigated by the Naval Criminal Investigative Service at the time on suspicion of perjury but refused to answer questions, and the matter was ultimately dropped.

But when a Washington Post story published earlier this year revealed new evidence that Thompson had lied to an administrative board of inquiry about having a sexual relationship with midshipmen, investigators approached Pretus with an offer: Testify for the prosecution against Thompson and enjoy protection from prosecution for his own role in the illicit behavior.

Thompson is set to face court-martial in January on charges of making false official statements to a board of inquiry that voted to keep him in the Marine Corps after his first trial; and of conduct unbecoming an officer and gentleman for allegedly lying to the Washington Post reporter, John Woodrow Cox, about the nature of his relationship with the two midshipmen.

In a motions hearing Sept. 13 at Marine Corps Base Quantico, Virginia, defense attorney Navy Lt. Clay Bridges said the three attorneys for the prosecution engaged in misconduct by not allowing Pretus to confer with an attorney before signing the immunity offer, made by Vice Adm. Walter E. "Ted" Carter, superintendent of the Naval Academy.

At the time of the offer, Pretus made a lengthy statement in which he described his friendship with Thompson, the sexual encounter with a midshipman, and phone calls between the two on another night, in which he alleges Thompson revealed his intent to have sex with two drunken midshipmen then at his house.

"The actions of the trial team have resulted in a different testimony," Bridges said in Wednesday's motions hearing. "And we will never be able to get the testimony back."

It's not clear how Pretus' testimony might have been different if he had been allowed to confer with an attorney, as he requested. Military.com left a phone message for Pretus requesting comment but did not immediately receive a response.

Bridges also called into question the circumstances by which Pretus and Thompson allegedly committed similar offenses, but with very different outcomes.

"Why has he never had any negative action, that we have seen, taken against him?" he asked. "What's lost is the due process Maj. Thompson had."

Bridges requested the military judge, Lt. Col. Christopher Greer, dismiss the charges or put forth a lesser remedy for the "chilling" effect of Pretus' testimony, allegedly obtained inappropriately. Greer has yet to rule on the motion; he said there is no deadline to render his decision.

The lead attorney for the prosecution, Marine Maj. Babu Kaza, maintained there was no prosecutorial misconduct or violation of rights, and added that nothing done by the prosecution had affected the issues at the heart of the case.

"The defense spends a lot of time talking about issues essentially irrelevant to this court," he said. "It never asserts the statement is false."

Bridges said Wednesday that the defense was also considering the possibility of alleging unlawful command influence, a claim that Carter used his position of authority to influence perception within the Marine Corps of Thompson and his case ahead of his trial.

On Monday, The Washington Post reported that the academy has taken steps to strengthen its instructor vetting process in the wake of publicity surrounding the alleged misconduct of Thompson and Pretus.

"It's a very unique privilege to come teach at the U.S. Naval Academy... We want to make sure we're getting what's been advertised," Carter told the paper.

-- Hope Hodge Seck can be reached at hope.seck@military.com. Follow her on Twitter at@HopeSeck.